I'm currently useing "The Rake" - he's a rare spawn found outside of Thunderbluff in Mulgore. You can simply do an Allakhazam or Thottbot search for him. I was extraordinarily lucky enough to just stroll through the Barrens (only got ganked once) and then to Mulgore and immediately found him!
His main claim to fame is a 1.2 attack speed which is higher than any other pet for many levels. Ultimately, the only better pets are guys like "Broken Tooth" who is a rare spawn elite in Badlands or that uber-bat found in ZG. Furthermore, The Rake is very unique looking being that he's a Lion.
The benefits of a low attack speed is that they kill Casters. I utterly hate having a low attack speed pet sic'd on my lvl 60 mage. It's hard to cast while getting interrupted every second. So, in short - good stuff for PvP.
Also for PvP - You might also want to look up pets who are particularly fast (meaning in terms of pursuit speed). But I think that most of these are higher level as well. There used to be a rare spawn in Barrens (I looked for him too btw, but he wasn't around) that is pretty fast, but all the reports state that his speed was nerfed recently in a patch. The benefit to this is that they can chase down enemies. Look up the Frostwolves' from Alterac Valley for an example. These guys can out-run epic mounts. ---> Instant death for a cloth wearer to get chased down and then stunned cause you were attacked from behind only to receive a nice Aimed Shot Crit from a hunter.
Other than that, you might want to consider a good tanking pet if you're just grinding. These would be Bears or Turtles if I understand correctly.
The best source IMO for further info are the stickied threads on the WoW Hunter forums. There are some links there that further detail all the above, telll you where to find pets, and describe the pet abilities and such.
Good Luck !!!
